2D/3D Holographic Display System

ABSTRACT

A display system ( 300 ) comprising an optical system and a processing system. The optical system comprising a spatial light modulator ( 380 ), a light source, a Fourier transform lens, a viewing system ( 320, 330 ) and a processing system. The spatial light modulator is arranged to display holographic data in the Fourier domain, illuminated by the light source. The Fourier transform lens is arranged to produce a 2D holographic reconstruction in the spatial domain ( 310 ) corresponding to the holographic data. The viewing system is arranged to produce a virtual image ( 350 ) of the 2D holographic reconstruction. The processing system is arranged to combine the Fourier domain data representative of a 2D image with Fourier domain data representative of a phase only lens to produce first holographic data, and provide the first holographic data to the optical system to produce a virtual image.

The present invention relates a display system and a method ofdisplaying images. Embodiments relate to virtual image display systemsand methods, and some embodiments relate to head-up display systems.

BACKGROUND

Light scattered from an object contains both amplitude and phaseinformation. This amplitude and phase information can be captured on,for example, a photosensitive plate by well known interferencetechniques to form a hologram comprising interference fringes. Thehologram may be reconstructed to form an image, or holographicreconstruction, representative of the original object by illuminatingthe hologram with suitable light.

Computer-generated holography may numerically simulate the interferenceprocess using Fourier techniques.

It has been proposed to use holographic techniques in a two-dimensionalimage projector.

Referring to FIG. 1, there is shown a light source 100 which applieslight via a Fourier lens (120) onto a spatial light modulator (140) inthis case as a generally planar wavefront. The spatial light modulatoris reflective and consists of an array of a large number ofphase-modulating elements. Light is reflected by the spatial lightmodulator and consists of two parts, a first specularly reflectedportion (known as the zero order) and a second portion that has beenmodulated by the phase-modulating elements to form a wavefront ofspatially varying phase. Due to the reflection by the spatial lightmodulator all of the light is reflected generally back towards the lightsource (100) where it impinges on a mirror with aperture (160) disposedat 45° to the axis of the system. All of the image part of the light isreflected by the mirror towards a screen (180) that is generallyparallel to the axis of the system. Due to the action of the Fourierlens (120), the light that impinges on the screen (180) forms a realimage that is a reconstruction of an image from which the informationapplied to the phase modulating elements was derived.

Embodiments relate to an improved 2D real-time projector for formingvirtual images of holographic reconstructions and providing adaptivepositional control of the virtual image in space, and allow for spatialfiltering of the reconstruction.

SUMMARY OF THE INVENTION

Aspects of the invention are defined in the appended independent claims.

In summary, a spatial light modulator (SLM) forms an array ofphase-modulating elements that collectively represent a phase-onlyFourier transform of a desired image which can be reconstructed bycorrectly illuminating the SLM, to form a projector. The phase-onlydistribution may be referred to as a hologram. The image may bedescribed as the holographic reconstruction. The elements of the SLM maybe referred to as pixels.

The holographic reconstruction is imaged by an optical viewing system toform a virtual image. The inventor has recognised that by providingvariable lensing data to the hologram, the position of the virtual imagerelative to a viewer can be changed. This can provide a “depth” to thedisplay system and allow virtual images to be presented at differentdistances from the viewer to provide a pseudo 3D system in real-time. Inparticular, the inventor has recognised that by forming an intermediatereconstruction, spatial filtering may be performed to remove higherdiffracted orders produced by the hologram. This gives rise to animproved viewing system particular for real-time applications such ashead-up displays.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described to theaccompanying drawings in which:—

FIG. 1 shows the basic principle of a conventional holographic imagedisplay;

FIG. 2 shows a schematic drawing of an example of a reflective SLM;

FIG. 3 shows a schematic drawing of a display;

FIG. 4 shows the effect of varying lensing information at the positionof the virtual image.

FIG. 5 shows a schematic drawing of a LCOS SLM device;

In the figures like reference numerals referred to like parts.

DETAILED DESCRIPTION OF THE DRAWINGS

It is found that the phase information alone is sufficient to generate ahologram which can give rise to a holographic reconstruction ofacceptable quality. That is, the amplitude information in the hologramcan be discarded. This can reduce the power of the required laser lightsources but has other advantages too. Fourier-based computer generatedholographic techniques have therefore been developed using only thephase information.

The image reconstructed by a hologram is given by the Fourier transformof the hologram. The hologram is therefore a phase-only patternrepresentative of the Fourier transform of the object whereas thereconstructed image (or holographic reconstruction) may contain bothamplitude and phase information.

Gerchberg-Saxton is one example of an iterative algorithm forcalculating a phase only hologram from input image data comprising onlyamplitude information. The algorithm starts from a random phase patternand couples this with amplitude data to form complex data. A discreteFourier transform is performed on the complex data and the resultantdataset is the Fourier components, which are made up of magnitude andphase. The magnitude information is set to a uniform value, and thephase is quantised, to match the phase values available. An inversediscrete Fourier transform is then performed. The result is anothercomplex dataset, where the magnitude information is overwritten by thetarget image and the process is repeated. The Gerchberg-Saxton algorithmtherefore iteratively applies spatial and spectral constraints whilerepeatedly transferring a data set (amplitude and phase), between thespatial domain and the Fourier (spectral) domain.

The Gerchberg-Saxton algorithm and derivatives thereof are often muchfaster than other “non-Fourier transform” algorithms such as directbinary search algorithms. Modified algorithms based on Gerchberg-Saxtonhave been developed—see, for example, co-pending published PCTapplication WO 2007/131650 incorporated herein by reference.

These improved techniques are able to calculate holograms at asufficient speed that 2D video projection is realised. Embodimentsdescribed herein relate to 2D video projection using acomputer-generated hologram calculated using such a modifiedGerchberg-Saxton algorithm

Holographically generated 2D video images are known to possesssignificant advantages over their conventionally projected counterparts,especially in terms of definition and efficiency. However, thecomputational and hardware complexity of the current hologram generationalgorithms preclude their use in real-time applications. Recently theseproblems have been solved—see, for example, published PCT application WO2005/059881 incorporated herein by reference.

To display the phase only holographic data, a phase modulating device isrequired. Since these devices do not modulate the amplitude, they areoptically transparent, in general. Therefore no light is lost toabsorption, for example. This has the major advantage that all of thereconstruction light is used in the creation of the holographicreconstruction. This translates to a more energy efficient displaysystem.

The phase modulating device may be pixellated and each pixel will act asa diffractive element. The diffraction pattern from each pixel will giverise to a complex interference pattern at a screen referred to as areplay field. Due to this complex relationship, each pixel on thehologram contributes to multiple parts of the reconstructed image.

An example phase modulating device is a spatial light modulator (SLM).Typically a SLM has a field of addressable phase-modulating elements. Insome SLMs the phase-modulating elements are a linear or one-dimensionalarray of elements; in others a two dimensional array are provided. Forsimplicity many SLMs have a regular 2-D array of like, generally square,phase-modulating elements; it is however not necessary for thephase-modulating elements to be alike in size or shape.

FIG. 2 shows an example of using a reflective SLM, such as a LCOS, toproduce a holographic reconstruction at a replay field location, inaccordance with the present disclosure.

A light source (210), for example a laser or laser diode, is disposed toilluminate the SLM (240) via a collimating lens (211). The collimatinglens causes a generally planar wavefront of light to become incident onthe SLM. The direction of the wavefront is slightly off-normal (i.e. twoor three degrees away from being truly orthogonal to the plane of thetransparent layer). The arrangement is such that light from the lightsource is reflected off a mirrored rear surface of the SLM and interactswith a phase-modulating layer to form an exiting wavefront (212). Theexiting wavefront (212) is applied to optics including a Fouriertransform lens (220), having its focus at a screen (225).

The Fourier transform lens receives light from the SLM and performs afrequency-space transformation to produce a holographic reconstructionat the screen (225) in the spatial domain.

In this process, the light from the light source is generally evenlydistributed across the SLM (240), and across the phase modulating layer.Light exiting the phase-modulating layer may be distributed across thescreen. There is no correspondence between a specific image region ofthe screen and any one phase-modulating element.

Referring to FIG. 3, there is shown an embodiment in accordance with thepresent disclosure using the SLM based system described above. FIG. 3shows a head-up display (300) having an SLM based system (305) forproviding a real image of a holographic reconstruction (310). Theholographic reconstruction is formed at a so-called replay field. Thespatial position of the replay field may be varied in accordance withembodiments described herein.

The display consists of an optical combiner (320) and a lens (330)disposed between the holographic reconstruction (310) and the combiner(320). The arrangement is such that a viewer (340) looking towards thecombiner (320) will see a virtual image (350) of the holographicreconstruction (310) at a distance d from the viewer and behind thecombiner (320). Such a system can be used for example in a head-updisplay or head-mounted display.

The optical system (335) may consist of a lens having a focal length f,and located at distance ed from the viewer. The holographicreconstruction (310) is at a real distance od behind the lens. If theholographic reconstruction (310) is disposed in the focal plane of thelens (330) then the viewer (340) will perceive the image (350) to be atinfinity.

However if the holographic reconstruction (310) is closer to the lens(330) then the focal length of the lens (330) the image (350) will nolonger be at infinity.

Provided the holographic reconstruction is closer to the lens (330) thanthe focal length of the lens then the image (350) can be arranged toappear closer than at infinity, and appear at a virtual distance vd. Thecalculations are as follows:—

${od} = \frac{1}{\left( {\frac{1}{f} - \frac{1}{- {vd}}} \right)}$

The above-mentioned replay field location may be varied by varying thelensing characteristic of phase only lensing data applied to the spatiallight modulator (380). Thus for a first lensing characteristic theposition of the real image (310) can be relatively close to the lens(330) and for a second value of lensing data the real image 310 isrelatively more distant from the lens (330). This means that the image(350) created by the virtual image display can be varied in apparentdepth.

In summary the information that is applied to the phase modulatingelements of the SLM (380) consists of two parts, a first part thatcomprises the information representative of the final image and a secondpart which has the effect of providing a negative lensing and adjustmentcharacteristic. By varying this latter part it is possible to cause theposition of the holographic reconstruction and therefore virtual image(350) to be varied.

By using a sufficiently fast spatial light modulator, an appropriatecomputational algorithm and by writing data appropriately to the spatiallight modulator it is possible to image different sub-frames of data atdifferent apparent depths. The SLM must be sufficiently fast to allowinformation to be electrically written and optically read-out multipletimes in a standard video frame. If the sub-frames are displayedsufficiently quickly, they may appear to a human viewer to be presentsimultaneously.

For example a general image can appear to be 2.5 metres from the viewer(340) but a part of the image—for example an image of especialimportance to the viewer—can, by providing its imaging data in adifferent sub-frame and by changing the lensing data for thatsub-frame—cause that image to appear in front of the general imageplane. This is shown schematically in FIG. 4 which shows four differentsubframe image positions, denoted 501, 502, 503 and 504.

The arrangement of FIG. 3 should be distinguished from configurations inwhich the viewer is positioned at the real image (310). Suchconfigurations may be referred to as “direct view”. In such cases, theviewer's eyes as the Fourier lens.

In summary the present disclosure relates to a virtual image display inwhich a holographic reconstruction (310) is first formed as a real imagein space. The real image (310) forms the “object” for lens (330) whichproduces a virtual image (350) of the real image (310). The virtualimage (350) may be seen by the viewer by looking through the opticalcombiner (320) as shown in FIG. 3.

By modifying the lensing data applied to the spatial light modulator(380), the position of the real image (310) can be changed. Accordingly,the position of the virtual image (350) can also be changed.

In contrast, when the viewer is position at the real image (310) theviewer functions as the Fourier lens and so sees all diffracted ordersof the reconstruction field. That is, the viewer would see multiplereplicas of the primary reconstruction—in order words, multiplereconstructions. The presence of multiple orders may lead to confusionparticularly in a head-up display, for example.

Additionally the quality of the reconstructed hologram is also affect bythe so-called zero order problem which is a consequence of thediffractive nature of the reconstruction.

Such zero-order light can be regarded as “noise” and includes forexample specularly reflected light, and other light that is unrefractedby the patterns on the spatial light modulator.

This “noise” is generally focused at the focal point of the Fourierlens, leading to a bright spot at the centre of a reconstructedhologram. In a direct view application the zero order would be asubstantial distraction when looking at the virtual image.

Advantageously, by imaging the intermediate reconstruction it ispossible to filter out the zero order and the higher diffracted ordersof the reconstruction field at the intermediate reconstruction. This maybe achieve, for example, by positioning a spatial filter at the realimage (310) to provide a physical aperture through which only preferredorders such as the primary order can pass.

Conventionally, the zero order light is simply blocked out however thiswould clearly mean replacing the bright spot with a dark spot.

However as the hologram contains three dimensional information, it ispossible to displace the reconstruction into a different plane inspace—see, for example, published PCT application WO 2007/131649incorporated herein by reference.

The application of the present invention includes head up displays andhead mounted displays, inter alia. The invention allows for full colourholograms with different information at different distances or depthsfrom the viewer, full 3D with a very limited volume by stacking multiplesub-frames, a large number of different images at different distances,perspective tracking of objects and enhance reality, for example anear-eye augmented-reality system with the ability to overlay differentinformation at different depths.

In embodiments, the spatial light modulator is a Liquid Crystal oversilicon (LCOS) device. The image quality is, of course, affected by thenumber of pixels and the number of possible phase levels per pixel.

LCOS devices are a hybrid of traditional transmissive liquid crystaldisplay devices, where the front substrate is glass coated with IndiumTin Oxide to act as a common electrical conductor. The lower substrateis created using a silicon semiconductor process with an additionalfinal aluminium evaporative process being used to create a mirroredsurface, these mirrors then act as the pixel counter electrode.

Compared with conventional glass substrates these devices have theadvantage that the signal lines, gate lines and transistors are belowthe mirrored surface, which results in much higher fill factors(typically greater than 90%) and higher resolutions.

LCOS devices are now available with pixels between 4.5 μm and 12 μm,this size is determined by the mode of operation and therefore amount ofcircuitry that is required at each pixel.

The structure of an LCOS device is shown in FIG. 5.

A LCOS device is formed using a single crystal silicon substrate (402).It has a 2D array of square planar aluminium electrodes (401), spacedapart by a gap (401 a), arranged on the upper surface of the substrate.Each of the electrodes (401) can be addressed via circuitry (402 a)buried in the substrate (402). Each of the electrodes forms a respectiveplanar mirror. An alignment layer (403) is disposed on the array ofelectrodes, and a liquid crystal layer (404) is disposed on thealignment layer (403). A second alignment layer (405) is disposed on theliquid crystal layer (404) and a planar transparent layer (406), e.g. ofglass, is disposed on the second alignment layer (405). A singletransparent electrode (407) e.g. of ITO is disposed between thetransparent layer (406) and the second alignment layer (405).

Each of the square electrodes (401) defines, together with the overlyingregion of the transparent electrode (407) and the intervening liquidcrystal material, a controllable phase-modulating element (408), oftenreferred to as a pixel. The effective pixel area, or fill factor, is thepercentage of the total pixel which is optically active, taking intoaccount the space between pixels (401 a). By control of the voltageapplied to each electrode (401) with respect to the transparentelectrode (407), the properties of the liquid crystal material of therespective phase modulating element may be varied, thereby to provide avariable delay to light incident thereon. The effect is to providephase-only modulation to the wavefront, i.e. no amplitude effect occurs.

A major advantage of using a reflective LCOS spatial light modulator isthat the liquid crystal layer is half the thickness that it would be ifa transmissive device were used. This greatly improves the switchingspeed of the liquid crystal (a key point for projection of moving videoimages). A LCOS device is also uniquely capable of displaying largearrays of phase only elements in a small aperture. Small elements(typically approximately 10 microns) result in a practical diffractionangle (a few degrees) so that the optical system does not require a verylong optical path.

It is easier to adequately illuminate the small aperture (a few squarecentimetres) of a LCOS SLM than it would be for the aperture of a largerliquid crystal device. LCOS SLMs also have a large aperture ratio, thereis very little dead space between the pixels (as the circuitry to drivethem is buried under the mirrors). This is an important issue tolowering the optical noise in the replay field.

The above device typically operates within a temperature range of 10° C.to around 50° C., with the optimum device operating temperature beingaround 40° C. to 50° C.

As a LCOS device has the control electronics embedded in the siliconbackplane, the Fill factor of the pixels is higher, leading to lessunscattered light leaving the device.

Using a silicon backplane has the advantage that the pixels areoptically flat, which is important for a phase modulating device.

A colour 2D holographic reconstruction can be produced and there are twomain methods of achieving this. One of these methods is known as“frame-sequential colour” (FSC). In an FSC system, three lasers are used(red, green and blue) and each laser is fired in succession at the SLMto produce each frame of the video. The colours arc cycled (red, green,blue, red, green, blue, etc.) at a fast enough rate such that a humanviewer sees a polychromatic image from a combination of the threelasers. Each hologram is therefore colour specific. For example, in avideo at 25 frames per second, the first frame would be produced byfiring the red laser for 1/75^(th) of a second, then the green laserwould be fired for 1/75^(th) of a second, and finally the blue laserwould be fired for 1/75^(th) of a second. The next frame would then beproduced, starting with the red laser, and so on.

An alternative method, that will be referred to as “spatially separatedcolours” (SSC) involves all three lasers being fired at the same time,but taking different optical paths, e.g. each using a different SLM, andthen combining to form the colour image.

An advantage of the frame-sequential colour (FSC) method is that thewhole SLM is used for each colour. This means that the quality of thethree colour images produced will not be compromised because all pixelson the SLM are used for each of the colour images. However, adisadvantage of the FSC method is that the overall image produced willnot be as bright as a corresponding image produced by the SSC method bya factor of about 3, because each laser is only used for a third of thetime. This drawback could potentially be addressed by overdriving thelasers, or by using more powerful lasers, but this would require morepower to be used, would involve higher costs and would make the systemless compact.

An advantage of the SSC (spatially separated colours) method is that theimage is brighter due to all three lasers being fired at the same time.However, if due to space limitations it is required to use only one SLM,the surface area of the SLM can be divided into three equal parts,acting in effect as three separate SLMs. The drawback of this is thatthe quality of each single-colour image is decreased, due to thedecrease of SLM surface area available for each monochromatic image. Thequality of the polychromatic image is therefore decreased accordingly.The decrease of SLM surface area available means that fewer pixels onthe SLM can be used, thus reducing the quality of the image. The qualityof the image is reduced because its resolution is reduced.

Embodiments implement the technique of “tiling”, in which the surfacearea of the SLM is further divided up into a number of tiles, each ofwhich is set in a phase distribution similar or identical to that of theoriginal tile. Each tile is therefore of a smaller surface area than ifthe whole allocated area of the SLM were used as one large phasepattern. The smaller the number of frequency component in the tile, thefurther apart the reconstructed pixels are separated when the image isproduced. The image is created within the zeroth diffraction order, andit is preferred that the first and subsequent orders are displaced farenough so as not to overlap with the image and may be blocked by way ofa spatial filter.

As mentioned above, the image produced by this method (whether withtiling or without) comprises spots that form image pixels. The higherthe number of tiles used, the smaller these spots become. If one takesthe example of a Fourier transform of an infinite sine wave, a singlefrequency is produced. This is the optimum output. In practice, if justone tile is used, this corresponds to an input of a single phase of asine wave, with a zero values extending in the positive and negativedirections from the end nodes of the sine wave to infinity. Instead of asingle frequency being produced from its Fourier transform, theprinciple frequency component is produced with a series of adjacentfrequency components on either side of it. The use of tiling reduces themagnitude of these adjacent frequency components and as a direct resultof this, less interference (constructive or destructive) occurs betweenadjacent image pixels, thereby improving the image quality.

Preferably, each tile is a whole tile, although it is possible to usefractions of a tile.

There is provided a method of displaying images comprising varyinglensing data on a spatial light modulator while varying imaging dataapplied to the spatial light modulator, whereby images of objects may beformed at different depths with regard to an image plane.

This image plane may be used in a virtual imaging system.

The step of varying data may be carried out in such a way that theplural images formed at different depths appear to the human eye to besimultaneously present.

There is provided a method of displaying, the method comprising applyingdata for forming an image to a SLM, illuminating the SLM, applying theresultant light to an optical system for forming a virtual image,wherein the data applied to the SLM includes first data and second data,the first data related to the content of the image and the second datadetermined to provide at least a lensing function by the SLM, the methodfurther comprising varying the second data in such a way that pluralimages formed at different depths appear to the human eye to besimultaneously present.

The method may comprise varying the first data whereby the plural imagesdiffer from one another.

There is provided a display comprising an SLM, circuitry for operatingthe SLM, an illumination device for illuminating the SLM and an opticalsystem adapted to form a virtual image reconstructed from data on theSLM, wherein the circuitry is adapted to apply data for forming an imageto the SLM, the data applied including first data and second data, thefirst data related to the content of the image and the second datadetermined to provide a lensing function by the SLM, and the circuitryadapted to vary the second data in such a way that plural images formedat different depths appear to the human eye to be simultaneouslypresent.

The optical system may comprise a Fourier lens

The display may form a head-up display.

The invention is not restricted to the described embodiments but extendsto the full scope of the appended claims.

1. A method for displaying a virtual image to a viewer, the methodcomprising: displaying first holographic data representative of a firsttwo-dimensional (2D) image and first holographic lensing datarepresentative of a first optical power on a phase-modulating spatiallight modulator; producing, as a real image in space spatially remotefrom the viewer, a first 2D holographic reconstruction image, bymodulating light from a light source with the spatial light modulator;performing a holographic transform on the light by way of a holographictransform lens; and displaying a first virtual image of the first 2Dholographic reconstruction image using an optical viewing system.
 2. Themethod of claim 1, wherein producing the first 2D holographicreconstruction image includes illuminating the spatial light modulatorwith the light to produce first spatially modulated light, thenperforming a holographic transform of the first modulated light.
 3. Themethod of claim 2, further comprising, after performing the holographictransform, spatially filtering the light to selectively block at leastone diffraction order of the first 2D holographic reconstruction image.4. The method of claim 1, further comprising: after displaying the firstholographic data representative of the first 2D image and the firstholographic lensing data on the phase-modulating spatial lightmodulator, displaying second holographic data representative of a second2D image and second holographic lensing data representative of a spatiallight modulator; producing, as a real image in space spatially remoteform the viewer, a second 2D holographic reconstruction image of thesecond 2D image by modulating the light with the spatial light modulatorand performing a holographic transform on the light; and displaying asecond virtual image of second 2D holographic reconstruction image usingthe optical viewing system, wherein the first virtual image and secondvirtual images are spatially displaced relative to each other to formreplay fields at different distances from a viewer.
 5. The method ofclaim 4, wherein producing the second 2D holographic reconstructionimage includes illuminating the spatial light modulator with the lightto produce second spatially modulated light, then performing aholographic transform of the second modulated light.
 6. The method ofclaim 5, further comprising, after performing the holographic transform,spatially filtering the light to selectively block at least onediffraction order of the first 2D holographic reconstruction image. 7.The method of claim 1, wherein the optical viewing system comprises animaging lens having a focal length, the lens being disposed at adistance less than the focal length from the position of the real imagein space, and wherein the displaying the first virtual image of thefirst 2D holographic reconstruction image comprises imaging the realimage with the imaging lens.
 8. The method of claim 1, wherein the firstvirtual image of the first 2D holographic reconstruction image isdisplayed on a heads-up display.
 9. The method of claim 1, wherein thereplay fields are spatially remote from the viewer.
 10. The method ofclaim 1, further comprising selectively blocking at least onediffraction order of the first 2D holographic reconstruction image. 11.The method of claim 1, further comprising selectively blocking a zerothundiffracted order of the first 2D holographic reconstruction image. 12.The method of claim 1, wherein the spatial light modulator comprises anarray of pixelated diffractive elements.
 13. The method of claim 12,wherein the spatial light modulator is a phase-only spatial lightmodulator.
 14. The method of claim 1, wherein the first lensing data isrepresentative of a negative optical power.
 15. The method according toclaim 1, wherein the first holographic data representative of the first2D image is Fourier-domain data representative of the first 2D image,the first holographic lensing data representative of the first opticalpower is Fourier-domain lensing data representative of the first opticalpower, and the holographic transform is a Fourier transform.
 16. Themethod of claim 1, wherein the holographic transform is performed by thefirst holographic-domain lensing data displayed on the phase-modulatingspatial light modulator.
 17. A display system for displaying a virtualimage of a first two-dimensional (2D) image to a viewer, the displaysystem comprising: a holography system configured to produce a first 2Dholographic reconstruction image of the first 2D image, the holographysystem comprising: a processing system configured to provide firstholographic data representative of the first 2D image and to providefirst holographic lensing data representative of a first optical power;a phase-modulating spatial light modulator configured to receive anddisplay the first holographic data representative of the first 2D imageand the holographic lensing data; a light source configured toilluminate the spatial light modulator, the holography system beingconfigured to modulate the light with the spatial light modulator; and aholographic transform lens configured to perform a holographic transformof the modulated light to produce the first 2D holographicreconstruction image; and an optical viewing system configured todisplaying a first virtual of the first 2D holographic reconstructionimage to the viewer.
 18. The display system of claim 17, wherein thelight source is configured to illuminate the spatial light modulator toproduce a first spatially modulated light.
 19. The display system ofclaim 17, wherein the optical viewing system further comprises a spatialfilter configured to selectively block at least one diffraction order ofthe first 2D holographic reconstruction image.
 20. The display system ofclaim 17, wherein the optical viewing system comprises an imaging lenshaving a focal length, the lens being disposed at a distance less thanthe focal length from the position of the real image in space, andwherein the imaging lens is configured to image the real image to formthe virtual image.
 21. The display system of claim 17, wherein the firstphase only lens has a negative optical power.
 22. The display system ofclaim 17, wherein the spatial light modulator is a phase-only spatiallight modulator.
 23. The display system of claim 17, wherein theprocessing system is further configured to provide second holographicdata representative of a second 2D image and to provide secondholographic lensing data representative of second optical power to thefirst optical power; the phase-modulating spatial light modulator isfurther configured to, after receiving and displaying the firstholographic data representative of the first 2D image and the firstholographic lensing data, receive and display the second holographicdata representative of the second 2D image and the second holographiclensing data; and the optical viewing system is further configured to,after displaying the first virtual image of the first 2D holographicreconstruction, display a second virtual image of the second 2Dholographic reconstruction image to the viewer, the first virtual imageand the second virtual image being displayed at different virtualdistances from the viewer.
 24. The display system of claim 23, whereinthe first virtual image and second virtual image are sequential framesof a 2D video stream, but appear to a viewer to be simultaneouslypresent.
 25. The display system of claim 17, wherein the firstholographic data representative of the first 2D image is Fourier-domaindata representative of the first 2D image, the first holographic lensingdata representative of the first optical power is Fourier-domain lensingdata representative of the first optical power; and the holographictransform is a Fourier transform.
 26. The display system of claim 17,wherein the first holographic data representative of the first 2D imageis Fourier-domain data representative of the first 2D image, and theholographic transform is a Fourier transform.
 27. The display system ofclaim 17, wherein the first holographic lensing data representative ofthe first optical power is Fourier-domain lensing data representative ofthe first optical power; and the holographic transform is a Fouriertransform.
 28. The display system of claim 17, wherein the system isconfigured to perform the holographic transform on the light using thefirst holographic-domain lensing data displayed on the phase-modulatingspatial light modulator.
 29. The display system of claim 17, configuredas a head-up display.